This invention relates to a surface profile measuring device, and more particularly to a surface profile measuring device utilizing optical heterodyne interference between two laser beams having different frequencies.
An optical heterodyne interference method has been conventionally utilized in a device for measuring a surface profile such as roughness of an object to be measured. In this method, a measuring beam outputted from a laser source is converged through an object lens on the surface of the object which is relatively movable in the direction intersecting to the optical axis of the measuring beam, and is reflected from the surface of the object while it is provided with information on the surface profile of the object. The reflected beam from the surface of the object is allowed to interfere with a reference beam having a different frequency from that of the reflected beam to form a composite beam. The composite beam is detected by an optical sensor and a measuring beat signal having information on the surface profile of the object is outputted therefrom. The surface profile of the object is measured on the basis of phase variation of the beat signal. In the conventional device for determing the surface profile, the object lens and the object to be measured are arranged so that the optical sensor provides an output signal having the largest amplitude.
However, the above conventional device for determing the surface profile of an object has a disadvantage that measurements obtained by the conventional device are not necessarily coincident with those obtained by another device adopting another principle or method, such as a device adopting a probe contact method. This method and the disadvantage thereof is described in "O plus E" published on April, 1988 by Shin Gijutsu Communications, Co., Ltd.